Dialog content transitions

* Implements fade(In/Out) transitions when switching
between view and edit/add mode
Metadata edit form

* Added a template for the form to be displayed when
a user wants to add a new metadata item via the edit
metadata dialog

* TODO: The template for adding place metadata
Add Drops API class

* Added stubs for the Drops API class

* Added SwiftRiver_Client#get_drops_api() to get and
return the Drops API
Add drop service class

* The drops service class invokes drop-related API requests
on behalf of the drop controllers